Cassop Pond in May

A month on from the phycological debauchery that I wrote about in Promising Young Algae the Spirogyra flocs that covered quite a lot of the surface of Cassop Pond have disappeared.  With sexual reproduction over, the zygotes, I presume, have sunk to the bottom of the lake, where they will lay dormant until next Spring.   I searched around the margins of the pond, but only found a few wisps of Spirogyra hanging around some cattle hoofprints in the shallow water on the eastern side of the pond.   This, however, proved to be a different type of Spirogyra altogether, with broader and squatter cells than the main constituents of April’s flocs.  

The green tinge in this hoofprint is filaments of Spirogyra.   The picture at the top of the post shows Cassop Pond in May 2021: note the absence of flocs compared with the situation in April. 

Swimming around amongst the Spirogyra filaments were a number of very active green cells of Euglena.   Euglena is a genus that has not featured much in this blog over the years (see: “A visit to Loughrigg Fell”) but it is a genus closely associated with Cassop Vale, with 13 of the 36 species recorded from Britain and Ireland recorded from this location.  The story behind this richness is that there were no active experts in the Euglenophyta at the time when the Freshwater Algal Flora of the British Isles was being compiled.   Instead, a Polish expert, Konrad Wołowski, was invited to contribute and, to help him do this, he was taken on a tour of locations that Dave John and Brian Whitton, the editors, thought would be likely habitats.  Cassop Pond is well known to Brian and is conveniently located near Durham, so it was an obvious location.  As a result, it is probably the hot spot of recorded diversity for this genus in the UK but that is more due to the idiosyncrasies of biological recording than to anything about this location over many others that particularly favours Euglena

That said, cattle hoof prints are known to be a good location for Euglena and relatives, and I wrote about a relative of Euglena that I found in a puddle in Teesdale (see: “Puzzling puddles on the Pennine Way …”.  A hoofprint or a puddle is, from an alga’s point of view, a temporary pond and so long as you have a plan in place for when this dries up, it represents a potential habitat.   Cattle, as we have already seen, are allowed to graze on the reserve so there are plenty of damp hoofprints within which Euglena and relatives can thrive.  That’s also true of many other nature reserves around the country.  The one missing ingredient at all of those is naturalists within an inclination to search them out …

Spirogyra filaments from Cassop Pond, May 2021.  Scale bar: 20 micrometres (= 1/50th of a millimetre).


Wołowki, K. (2010).  Euglenophyta.  pp. 181-239.   In: Freshwater Algal Flora of the British Isles (edited by John, D.M., Whitton, B.A. & Brook, A.J.). Cambridge University Press, Cambridge.

Euglena sp. from Cassop Pond, May 2021.  Scale bar: 10 micrometres (= 1/100th of a millimetre).  

Some other highlights from this week:

Wrote this whilst listening to:   The soundtrack to The Pursuit of Love, available via BBC Sounds.   

Cultural highlights:   Emily Mortimer’s adaptation of Nancy Mitford’s The Pursuit of Love, available on the BBC iPlayer

Currently reading:  Pat Barker’s Noonday, the final part of the Life Class trilogy.

Culinary highlight: homemade digestive buiscuits.

The diatoms of Cassop Pond

We’ll stay at Cassop Pond for this next post, as I look back over the diatoms that I’ve found there.   So far, I have collected four samples although, due to the time it takes to prepare these for analysis, I’ve only got around to looking closely at three of these. Nonetheless, I’ve found a total of 98 species belonging to 39 genera in these three samples.  Here is a summary of the more abundant forms.

Araphid diatoms were particularly abundant in the sample I collected from reed stems in January.  Despite my comments in the post I wrote at the time (see: “A winter’s tale …”), the most abundant are Tabularia fasiculataand Ulnaria cf. acus., both of which grow singly or in small clusters, attached to the stem by a mucilage pad at one end.  The genus Tabularia is often described as a species of brackish and marine waters, but in my experience, it can be abundant in freshwater habitats where the water is quite hard.  

Araphid diatoms from Cassop Pond, 2021: a., b. Tabularia fasiculata; c. Ulnaria acus; d. Fragilaria tenera; e. Fragilaria cf. pectinalis; d. Fragilaria, unidentified girdle views; g. Diatoma tenuis; h. Tabellaria flocculosa.   Scale bar: 10 micrometres (= 1/100th of a millimetre). 

Cocconeis species were particularly abundant in the sample from Riccia fluitans and Lemna minor collected in February.   This genus is often abundant as an epiphyte on other plants and algae and it is common to find more than one species in the same sample, which would suggest that there are some subtle aspects of their niches that we do not yet fully understand.   Two of these species were also encountered growing on rocks in Croasdale Beck in Cumbria (see “Curried diatoms?”).  Lemnicola hungarica was also present in the samples, albeit in low numbers.  This species is often epiphytic on Lemna minor (see: “The green mantle of the standing pond …”) and I suspect that a sample composed mostly of Lemna and with less Riccia fluitans might have a higher proportion.   Finally, I have included five different focal planes of a single valve of Eucocconeis flexella, just to show the complexity of valve structure in this diatom.   Note the S-shaped raphe on the upper valve.

Monoraphid diatoms from Cassop Pond, 2021.  a., b. Cocconeis euglypta; c. C. lineata; d.,e. C. pseudolineata; f. Achnanthidium caledonicum; g. A. eutrophilum; h. A. saprophilum; i., j., k.Planothidium lanceolatum (3 focal planes); l. Platessa oblongella; m., n., o., p., q, Lemnicola hungarica; r., s., t., u., v. Eucocconeis flexella.  Scale bar: 10 micrometres (= 1/100th of a millimetre). 

One of the surprises of this sample was the relatively high proportion of Eunotia species that I found, particularly in the sample from February.  Eunotia is a species most often associated with soft water so I had not expected to find it to be frequent in a calcareous pond.   However, this sample was collected on the east side of the pond, where some spoil heaps form part of the shoreline.   Moreover, neither of the two species that were most abundant are particularly associated with very low pH.  However, these were a curiosity and there were also a few other species in the samples (e.g. Tabellaria flocculosa) which hinted that soft water might have some influence in the pond.   

Eunotia species from Cassop Pond, 2021.   a.,b. Eunotia bilunaris; c.,d.,e.,f. E. minor; g. E. incisa.   Scale bar: 10 micrometres (= 1/100th of a millimetre). 

The role of Epithemia adnata in the pond was considered in Working their passage so we don’t need to say much more here except that this is another species that is far more abundant on plants than on rock surfaces in this pond.   By contrast, the genus Nitzschia was much more common the rocks at the north end of the pond than on the plants.   These rocks were covered with a fine layer of marl (fine calcite deposits that has precipitated out from the water).  The most abundant species here was Nitzschia palea.

Epithemia adnata from Cassop Pond, 2021.   Scale bar: 10 micrometres (= 1/100th of a millimetre).
Nitzschia and Tryblionella species from Cassop Pond, 2021. a. Nitzschia paleacea; b. N. subtilis; c., d., e. N. palea; f. N. cf. archibaldii; g. N. capitellata; h. Tryblionella sp.  Scale bar: 10 micrometres (= 1/100thof a millimetre).

Another diatom that was common on the rocks at the north end was Cymatopleura solea.  Whilst this was less abundant in terms of numbers than Nitzschia palea, it has much larger cells, so the overall contribution to biomass and photosynthesis is probably the same or even greater than that species.   When I tried to describe Cymatopleura solea, with its central  constriction along with transverse undulations across the valve surface, to a class a few years ago, one participant suggested that it was a “voluptuous” diatom.   The presence of this along with the Nitzschia suggests that motility is an attribute that favours diatoms in this habitat, in contrast to the two samples from plants, which were both dominated by non-motile species. 

Cymatopleura solea from Cassop Pond, 2021.   One valve photographed at three focal planes.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

The most diverse genus encountered was Navicula, with 14 species, although these were never found in great numbers.  As befits a motile genus, these were most abundant in the sample from the rock although they were also found in small numbers in the samples from plant surfaces.  Other biraphid symmetrical species found included Caloneis, Sellaphora, Hippodonta, Fallacia and Neidium.   Two of the images are described as “Sellaphora pupula” but we know that this is an aggregate of several species, barely distinguishable with the light microscope.  Both of these images are likely to represent different species. A significant omission from my list is Mastogloia (see “Structural engineering with diatoms”).  The habitat seems right for this species, and I have found it in other ponds in the area (see “Return to Croft Kettle”) so I suspect that it may turn up at some point during the year.

Biraphid symmetrical diatoms from Cassop Pond, 2021: a. Navicula cryptotenelloides; b., c. N. trivialis; d. N. subalpina; e. Caloneis amphisbaeana; f. Hippodonta capitata; g., h. Sellaphora pupula ag.; i. S .saugerresii; j. Fallacia pygmaea; k., l., m. Neidium dubium (three focal planes).  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Finally, there were a few valves of Gomphonema, Cymbella, Amphora and Halamphora, but none present in significant quantities.  

To put the 96 diatoms I’ve recorded to date into perspective, Heather found 182 and 123 angiosperm species over the course of a year at two nearby nature reserves, both with similar geology to Cassop Vale.   That puts the diversity of the microscopic world into perspective.  Bear in mind, too, that the samples I’ve looked at to date were collected in the winter.  I fully expect the final count of diatoms to exceed that of angiosperms but we’ll have to wait and see.   Is an element of competition creeping into this natural history malarky?   Surely not …

Heteropolar and dorsiventral diatoms from Cassop Pond, 2021: a. Gomphonema cf. graciledictum; b. Gomphonema sp.; c. Cymbella affinis; d. Halamphora montana.  Scale bar: 10 micrometres (= 1/100thof a millimetre).


Mann, D.M., Thomas, S.J. & Evans, K.M. (2008).  A revision of the diatom genus Sellaphora: a first account of the larger species in the British Isles.  Fottea (Olumec) 8: 15-78.

Some other highlights from this week:

Wrote this whilst listening to:   Scottish singer-songwriter Karine Polwart, who I first encountered as the “hold” music on Triodos Bank’s customer service line.  The first and only time in my life I wish I was 8th in the queue, instead of 6th.

Cultural highlights:   Nomadland, winner of the Oscar for best film.   A Ken Loach vibe but set in the western USA rather than north east England.

Currently reading:  John le Carré’s Absolute Friends.

Culinary highlight: our first meal out for many months, at Whitechurch in Durham.  Apart from the cold and the damp, it was great.  

Working their passage …

My February visit to Cassop Pond was a warmer experience than my January visit, though copious rain meant that the footpaths we followed to reach the reserve were extremely muddy.  I’ve been gazumped in my endeavour to bring you the phycological riches we encountered on this trip by Heather, who has already mentioned the two species of Trentepohlia that we noticed growing on the wall of an abandoned quarry in her post about the visit.  However, I still managed to find plenty to amuse me in the pond itself. 

On first glance, the pond still does not look very prepossessing.  The ice that had covered the pond during our January visit had gone, but there were still few signs of life above the water level.  It is only when we look below the surface that we find evidence of activity.   Standing at the edge of a marshy area, churned up by the herd of highland cattle who roam the reserve, I can see greenish flocs which, from a distance, they look like filamentous algae.  Up close, however, they are a tangle of narrow, branched strap-like fronds.   This is Riccia fluitans, one of just two free-floating liverworts found in Britain (the other one, Ricciocarpus natans, is also found here, but that’s a story for another day).   In my photograph, you’ll also see a few leaves of Lemna minor, duckweed.  That will become way more prolific later in the year.  Today, Riccia fluitans is the star of the show.

Flocs of the liverwort Riccia fluitans floating just below the surface in Cassop Pond, February 2021.
A close-up of the floc of Riccia fluitans in Cassop Pond.  Each frond is about a millimetre wide.

When we move in even closer and peer at Riccia fluitans through a microscope, the reason for its buoyancy becomes clear: there are a number of floatation chambers.  This, in turn, creates the impression that the cells that contain chloroplasts are forming sinuous bands along the thallus.   I had never thought of chloroplasts as being particularly heavy, but many aquatic plants are parsimonious in the way that they deploy their photosynthetic arsenal.  We saw this in the moss Sphagnum (see “Back to the bog”) and also, last year, in Potamogeton polygonifolius (see “The dark side of the leaf …”)

The thallus of Riccia fluitans viewed under a microscope, showing the mix of green photosynthetic cells and empty cells which help it to remain close to the surface. The thallus is about half a millimetre across.

One other characteristic that Riccia fluitans shares with Potamogeton polygonifolius (and, for that matter, Lemna minor: see “Cassop”) is that it carries passengers.   Cranking up the magnification on my microscope, I can just make out the outlines of several cells of the diatom Epithemia.  I’ll write more about the diatoms from Croft Kettle at a later date but Epithemia is an interesting find here because it is a diatom that is capable of nitrogen fixation (see “More about Croft Kettle”).  I’ve recorded Epithemia from Cassop Ponds in the past, but never noticed its affinity for Riccia fluitans before.   Rooted plants can draw on nitrogen in the sediments to meet their needs; however, free-floating plants depend on what can be acquired from the water.  We also know now that many aquatic organisms are quite leaky so an alga that has a trick like nitrogen fixation up its sleeve may be serving as a food bank for the host organism.   It might just be that the buoyancy that Riccia fluitans provides is a quid pro quo for these nutrients.   This looks like being a mutually beneficial relationship: this small corner of nature being less “red in tooth and claw” than “you scratch my back, I’ll scratch yours” …

Epithemia species just visible on the lower surface of Riccia fluitans from Cassop Pond.  The two cells at the centre of the image are each about a tenth of a millimetre long. 

Some other highlights from this week:

Wrote this whilst listening to: Kin Sonic by Congolese musicians Jupiter & Okwess.   

Cultural highlights:  The Martian, which is basically about growing potatoes in inhospitable environments.  “Fear my botany powers, Mars!”

Currently reading:  My Last Supper: One Meal, a Lifetime in the Making by Jay Rayner.  

Culinary highlight: Persian kababs in a meal box from Berenjak’s restaurant in London.  

And the Oscar for best alga in a supporting role goes to …


I know that the focus of this blog can meander, depending on what takes my fancy week-to-week.  My core business is, however, writing about the hidden world of algae so, having written about Sam Mendes’ use of the River Tees Upper in his film 1917 in my previous post, I thought that I ought to take a trip up Teesdale to take a closer look at what is growing in the river at this time of year.   With Storm Ciara looming ominously on the forecast, I knew that if I did not sacrifice my Saturday morning it might be a while before I had another opportunity (there’s a graph at the end of this post which confirms this hunch).  And so I found myself buffeted by the wind with clouds scudding across the sky and the peaty water of the Tees thundering across the sequence of cascades that make up Low Force.

The main river was, even after a period without much rain, too deep and fast-flowing for me to venture far in so my activities were confined to the margins.   The rapid current, however, means that there were few of the small and medium-sized stones that I would normally remove and inspect.  Most had been picked up and transported further downstream leaving wide expanses of the Whin Sill bedrock.   In the shallow areas towards the edges that were not exposed to the full force of the current, there were dark green patches that I picked at with a pair of forceps.   When I was able to look at these under my microscope, I saw that they were Ulothrix zonata, a common inhabitant of northern British streams during the winter, and an alga that I have written about previously (see “The intricate ecology of green slime …” and “Bollihope Bhavacakra” amongst others).


Ulothrix zonata growing on Whin Sill in the River Tees at Low Force, Teesdale in February 2020.   The upper and central pictures on the left hand side show vegetative filaments and the lower picture shows empty cell walls after zoospores had been released, to which a germling is attached.  Scale bar: 20 micrometres (= 1/50thof a millimetre).   

The rocks were very slippery, even when not covered by green patches of Ulothrix zonata.   My usual approach to collecting specimens is to remove the whole stone and scrub the top surface with a toothbrush.  That, however, was impossible here so I had to resort to brushing the surface of the Whin Sill and hoping that enough of the slippery film remained attached to my toothbrush, which I then agitated in a bottle containing some stream water to shake the gunk off before repeating the process.  The small amount of material that I did manage to transfer from the rocks imparted a chocolate-brown hue to the water that signifies that diatoms were present.

Sure enough, when I did get a drop of the suspension under my microscope, there were diatoms aplenty, mostly wedge-shaped cells of Gomphonema growing at the end of long, branched mucilaginous stalks.  These, like Ulothrix zonata, are very common in northern British streams at this time of year.  I described similar assemblages from the River Wear at Wolsingham although, in that case, the Gomphonema shared their habitat with motile Navicula species as well (see “The River Wear in January”).   The Gomphonema in the River Tees is most likely G. olivaceum or a relative but I will need a closer look to be sure.  If I used an old Flora such as Hustedt’s 1930 Süsswasser-flora Mitteleuropas, I would have been able to be more assertive in naming this “Gomphonema olivaceum” but we now know that diatom systematics are more complicated than was thought to be the case in Hustedt’s days.


Gomphonema olivaceum-type colonies growing on Whin Sill in the River Tees at Low Force, Teesdale, February 2020.  Scale bar: 20 micrometres (= 1/50th of a millimetre).   

The sequences of 2017 were filmed in June not January so George Mackay would not have found the bedrock of the Tees to be quite as slippery as it was on my visit.   As the water warms up, grazers become more active and, as a result, the biofilms in the summer are much thinner than those in January.  That means that fewer slippery, slimy polysaccharides are produced, making it easier to keep your balance when walking at the edges of the river.

As I mentioned in my previous post, the sequence in 1917 involves George Mackay falling into a river in Picardy but crawling out of a river in Upper Teesdale.   I know less about the rivers of Picardy than I do about those in northern England, but a combination of low relief, extensive canalisation and the presence of heavy industry and coal mining in the area will mean that the algae found there will be very different to those in the Tees.   However, if 1917 can get 10 Oscar nominations (including for best sound editing) despite having the call of a Great Northern Diver echoing over No-man’s Land, then we can be fairly sure that the Wrong Sort of Algae is a level of detail that Sam Mendes and Roger Deakins thought they could safely ignore.


You can find some information about the diatoms of Picardy rivers in this paper:

Prygiel, J. & Coste, M. (1993).  The assessment of water quality in the Artois-Picardie water basin (France) by the use of diatom indices.  Hydrobiologia 269: 343-349.

This week’s other highlights:

Wrote this whilst listening to:  Michael Kiwanuka and other acts who will be playing at the Green Man festival in August.   I’ll be there too, talking about slimy algae, at Einstein’s Garden, the on-site science festival, along with (I hope) a gang of volunteers from the British Phycological Society.

Cultural highlight:   Two picks this week.  The first was Monteverdi’s Vespers performed at Durham Cathedral.  The cavernous interior of the cathedral joins the choir and orchestra as part of the experience, providing resonances that raise the experience beyond anything that a CD can offer.   The second is Bong Joon-ho’s film Parasite, a strong contender, along with 1917, at this evening’s Oscar Awards Ceremony.

Currently reading:  John le Carré’s Mission Song

Culinary highlight: a Napoli pizza cooked with locally-grown flour (, part of a push this year to source more of our ingredients locally.  There’s obviously more to a Napoli pizza than can be grown in the UK but it is a start.


River levels at the Tees at Middleton-in-Teesdale (x km downstream from Low Force) in the week from 3 to 9 February 2020. The arrow shows the time of my visit; note the steep rise in level a few hours later, coinciding with Storm Ciara moving through the region.  Graph from the excellent website.

1917 and all that


If you haven’t seen it already, Sam Mendes’ film 1917 is well worth a trip to the cinema, particularly for the way it appears to have been filmed as a single take, which gives it a very immersive view of the brutality of trench warfare.   The result is a sense that we, the audience, are there alongside the protagonists that lasts up until the point when a small detail intrudes to break the spell and you find yourself sitting up and wondering what is going on.   This may be, I admit, a niche concern but, for me, this happens at the point at which the lead character, played by George Mackay, jumps into a quiet, slow-flowing canalized Picardy river to escape a pursuer.   As he enters the water, it transmogrifies into a fast-flowing rapid-strewn torrent, through which he struggles to keep his head above water until eventually crawling out onto rocks beside some woodland and, from there, back into Picardy and the trenches.

Was it just me, or was that an outcrop of Whin Sill in the background as Mackay thrashes around trying to keep his head above water?   I am no geologist but I had passed signs warning walkers that filming was taking place as I drove up Teesdale a few months ago and the rumours were that this was a location for a major film.  I guess even a geological dunce can start to recognise geological formations when he has seen the paraphernalia of a film set at precisely the point on the River Tees where Whin Sill outcrops create a cascade waterfall.


Filming 1917 at Low Force, Teesdale, June 2019.   The photo at the top shows Low Force on the River Tees (credit: Heather Kelly)

I am not alone.  A listener to Simon Mayo and Mark Kermode’s film programme on BBC Radio 5 wrote in to say that he had been discombobulated by the sound of a Great Northern Diver as, earlier in the film, Mackay and a fellow soldier had made their way across No Man’s Land.   The listener pointed out that the closest locations where the Great Northern Diver was found was northern Scotland, and that the haunting call of this bird appearing out of place had jarred with the film’s otherwise close attention to period detail.   A few years ago, I had a similar experience in a performance of Alan Bennett’s A Question of Attribution when a backdrop supposed to represent the Queen’s picture gallery at Buckingham Palace included Leonardo da Vinci’s Lady with an Ermine, which I knew to belong in a gallery in Krakow.   Such details can undo the hard work of cast and crew to create a particular atmosphere.   Instead of being immersed in the drama, you find yourself scratching your head and asking questions that the director had never anticipated.

As I said above, this is all very niche stuff.  A few ornithologists will have noticed the wrong birdsong (strange synchronicity to use that word in a post about trench warfare), almost everyone else will have benefited from the haunting call’s contribution to the overall atmosphere. And a very few river ecologists will have been disconcerted by the seamless transition from Picardy to Teesdale and back.  Everyone else (the vast majority) will have been grateful that Sam Mendes did not let Mackay float in a muddy, almost stationary French canal while German soldiers took potshots at him, and thus depriving the story of any sense of resolution.

There are, almost certainly, far more occasions when a wrong detail passes right over me than when I am discombobulated by something that I do notice.  Any kind of specialist knowledge will reveal small incongruities that a film crew did not realise mattered (they probably don’t in the grand scheme of things) or which were deliberately changed to heighten an effect (as Mendes did in 1917).   The few that notice will be temporarily sprung from the world of illusion that the director has striven to create but, for everyone else, the magic will be heightened.   We should, really, be grateful that Sam Mendes didn’t worry too much about a realistic depiction of the hydrology of Picardy.

This week’s other highlights:

Wrote this whilst listening to: Beethoven’s 9th Symphony. What else at this sad time?

Cultural highlight: Welsh harpist Caitrin Finch playing with Colombian band Cimarron at the Sage, Gateshead on Saturday night.

Currently reading:  The Memory Keeper’s Daughter by Kim Edwards

Culinary highlight: a sociable evening cooking Sichuan food with Heather and Rosie, recreating some of our favourite dishes from our trip to Chengdhu last year using Fuchsia Dunlop’s cookbook.   Consumed with two bottles of Durham Brewery’s Smoking Blonde ale

Reflections from Castle Eden Burn

As 2019 draws to a close, I have looked back at all the data I have collected from Castle Eden Burn over the past twelve months.   I chose this location precisely because it was different to my usual haunts and, despite having visited this Dene and others along the Durham coast for over thirty years, I realised that I had never had a look at the algae.  Dry river beds are not the most obvious hunting grounds for aquatic biologists, after all.   This year, I put that right over the course of a number of visits between January and November and in this post I am summarising what I found.

I found a total of 77 different diatoms in the six samples that I collected, not to mention green and yellow-green algae (see “When the going gets tough …”) and mosses (see “A thousand little mosses …”).   Of these diatoms, 48 were rare and infrequent, only found in one or two samples, and never forming more than one percent of the total number of diatoms present.   Of the remainder, only two were found in every sample (Humidophila contenta-type and Achnanthidium minutissimum) whilst another eight formed at least ten percent of the total on one occasion.  Numbers of each species waxed and waned over the year: Humidophila contenta-type was abundant in the sample from my first visit in January 2019 but relatively scarce thereafter.  In comparison, Luticola frequentissima was very abundant on two occasions (more than 80% of individuals), quite abundant on three other occasions but absent from the sample from my final visit in November.

Some of these differences are due to the variable flow regime: the stream was dry on three occasions, ponded on one and flowing on just two occasions.  Those occasions when there was no running water were those when the proportions of diatoms that are tolerant to desiccation (see “Life out of water …”) were most abundant, forming from 20 to 97 percent of all individuals.  When there was running water, it was motile Nitzschia  species that dominated.    In fact, there was a strong negative correlation between proportions of desiccation-tolerant and motile taxa in the samples, indicating that the diatoms responded rapidly to the changing pressures experienced in the stream.  There was also a relationship between the proportions of desiccation-tolerant diatoms and the number of taxa recorded – the latter is a good measure of the level of physiological stress experienced in a stream.

What of the diatoms themselves?  Humidophila contenta-type was one of the two ever-presents.  It is, however, very small (few of those in our samples were more than a 100th of a millimetre long), making it difficult to photograph and, indeed, to discern many of the features of the valve.   This species sometimes forms short chains though I did not see any in the Castle Eden Burn samples.  It is strange to think that, when I first started to identify diatoms, this was considered to be part of the genus Navicula.   Since then, it has moved into the genus Diadesmis before finally being transferred to the new genus Humidophila by Rex Lowe and colleagues in 2014.    Some recently-described Humidophila species cannot be differentiated from H. contenta without a scanning electron microscope, so I have referred to this as “Humidophila contenta-type”. Humidophila_contenta

Humidophila contenta ag. from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres.   Photograph: Lydia King. 

The most abundant diatom in samples collected during the dry periods was Luticola frequentissima.  I started the year referring to this as “Luticola mutica” but was gently corrected by colleagues more au fait with recent literature than me.   Luticola mutica is larger (length: 11-28 µm; breadth: 6-9.5 µm) and has more widely-spaced striae (16-18 / 10 µm) than L. frequentissima (length: 7 – 13.8 µm breadth: 4.8 – 6.8 µm; striae: 20 -24 / 10 µm).  The specimens in the plate below all fit the description for L. frequentissima.  Some of the large specimens have size ranges that overlap with L. mutica (though even the largest specimen as a striae density consistent with L. frequentissima).   L. mutica is associated with more brackish habitats whilst L. frequentissima prefers freshwaters.


Luticola frequentissima from Castle Eden Burn, Co. Durham, January 2019. Scale bar: 10 micrometres (= 1/100th of a. millimetre).  Photographs: Lydia King.

Simonsenia delognei is another characteristic species of habitats that dry out periodically.   This species, which is in the same family as Nitzschia, is quite small and only lightly silicified so easily overlooked.  It was common early in the year, but rare thereafter.  Whether this is a real characteristic of the species or an artefact of the conditions in Castle Eden Burn this year is difficult to tell as it is not a particularly common species so there are few other records against which this trend can be compared.


Simonsenia delognei from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres (= 1/100thof a millimetre). Photographs: Lydia King.

Two other species of Nitzschia were common: I illustrated N. clausii in “Out of my depth …” and have included photographs of N. sigma here.   I’m intrigued that two of the most conspicuous Nitzschia in this sample are sigmoid in outline.  I’ve visited the question of sigmoid diatoms before, and still don’t have any good explanation why a few diatoms have this outline (see “Nitzschia and a friend …”).  Note, too, that Nitzschia species can be sigmoid in valve view (i.e. looking down from above) or girdle view (i.e. looking from the side), although the great majority of species are straight in both planes.


Nitzschia sigma from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres (= 1/100th of a millimetre).   Photographs: Lydia King.

Finally, one more relative of Nitzschia that was found in a couple of samples, but never in large numbers, was Tryblionella debilis.  The genus Tryblionella was treated as part of Nitzschia for much of the 20th century.   As it appears to form a natural group with some distinctive characteristics, it is now generally treated as a distinct genus, although the molecular evidence indicates a complicated evolutionary history.   The principle characteristic of the genus is a longitudinal undulation on the valve face that is most clearly manifest on those species in the genus which have visible striae.   T. debilis is a small species with striae that are not resolvable with the light microscope; however, the undulations are just apparent as faint longitudinal lines running along the valve face.


Tryblionella debilis from Castle Eden Burn, Co. Durham, January 2019.  Scale bar: 10 micrometres (= 1/100th of a. millimetre).  Photographs: Lydia King.

That’s a lot of diatoms from a stream that is not always a stream.   I am sure that someone with interests in other groups of algae could probably make similarly long lists for some of those, and a more thorough exploration of habitats within the stream could add to the number of diatoms.  That’s before suggesting a molecular study, which might well reveal cryptic diversity (i.e. significant taxonomic variation that is impossible to discern with a light microscope) within the species I have already described.   The greater our capacity to unravel the mysteries of the microscopic world, the more, it seems, we discover we don’t know.


Lowe, R.L., Kociolek, P., Johansen, J.R., Van de Vijver, B., Lange-Bertalot, H. & Kopalová, K. (2014).  Humidophilagen. nov., a new genus for a group of diatoms (Bacillariophyta) formerly within the genus Diadesmis: species from Hawai’i, including one new species.  Diatom Research 29: 351-360.

Castle Eden Dene in November


For the first time this year, I heard Castle Eden Burn before I saw it.  Walking down from the car park, the distant roar of water was apparent almost as soon as the canopy of largely leafless branches closed over me.  A few trees still held their leaves – spectacularly golden on beech and birch, in particular, and the Dene’s famous yews were still green, of course – but the forest was dressed for winter now, much as it was on my first visit this year, back in January (see “Castle Eden Dene in January”).  Then, I was surprised that there was no water in the Burn.  On this trip, however, I wore my chest waders.  Back in August, I had compared Castle Eden Burn to a wadi (see “The presence of absence in Castle Eden Dene”) so the heavy rain of the previous few weeks had led me to suspect that today would be different.

The water surging through the Dene was very turbid, so collecting stones to examine involved feeling around on the river bed with my hand until I located one that was not sufficiently bedded into the substratum to remove.   That’s not ideal, but needs must and I got the five cobbles I needed, each with a distinct biofilm, slimy to the touch.  This is the first time, after eleven months, that Castle Eden Burn’s substratum has looked and felt remotely like the substratum from most of the other rivers I know in this part of the world.

Under the microscope, I see lots of particulate matter but also plenty of algae.   Apart from a few filaments of the cyanobacterium Phormidium, these were mostly diatoms.   The green algae I described in “When the going gets tough …” back in May were not obvious.  The diatoms were mostly largely motile cells of Navicula, with a few sigmoid cells of Nitzschia clausii and some smaller cells whose identity I will need to confirm once I have cleaned the sample and prepared a permanent slide.  The Navicula species, in particular, are typical inhabitants of local rivers during winter and early spring, all tolerant to a wide range of conditions.   I suspect that the rainfall has washed a lot of fine particulate debris from the industrial estates in the upper catchment into the river, and these diatoms will have the resilience to cope with such types of pollution.  A large storm sewer overflow also empties into the burn about a kilometre upstream of where I was standing and this, I suspect, has been flowing over the past month or two.

I also saw a few cells of Achnanthidium minutissimum, which I generally associate with cleaner conditions.  I suspect, however, that numbers will be relatively low compared to its more pollution-tolerant brethren.   Again, I can give a more authoritative answer once I have cleaned the sample and performed a full analysis.


Diatoms from Castle Eden Burn, November 2019.  a., b.: Navicula trpunctata; c. – e.: Navicula lanceolata; f., g.: Rhoicosphenia abbreviata; h., i.: Nitzschia clausii; j., k.: Navicula gregaria; l. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100thof a millimetre).   The photograph at the top of the post shows Castle Eden Burn just downstream from the point I sampled.

I originally set out to visit Castle Eden Burn six times during 2019 and this was the last of those. I’ve written about most of these visits already but not about my September visit.  There was, on that occasion, little new information to justify a separate post but I will include the sample I collected in my final overview of the algae of Castle Eden Burn, just as soon as I get this final sample cleaned and analysed.   Before then, I have one more post to write about the diatoms, based on some more detailed observations of a few of the species, and then it will be time to think about where to focus my observations during 2020.

The presence of absence in Castle Eden Dene


Some of my strongest impressions of Castle Eden Burn after last week’s visit concerned not what I found in the stream, but what was not there.  I mentioned in my previous post that I had not seen the mosses that I associated with streams in northern England in Castle Eden Burn, but there were other species, too, that I had expected to see but had not noticed.   Once I have noticed that something is absent, this absence becomes present.  I have noticed the presence of absence.  Woohoo: I’ve shoehorned Jean-Paul Sartre’s Being and Nothingness into a blog about ecology.

When I got back home I had read a chapter about the FBA’s study of the Winterbourne in Dorset, an intermittent stream flowing off the chalk downland, and noticed that they had recorded plants there that I knew from north-east English rivers, but which I could not remember seeing in Castle Eden Burn.  Was this because I had not searched the stream environs thoroughly, or is this a real difference between intermittent streams on chalk and on Magnesian limestone?

I went back this weekend to try to answer these questions, taking Heather with me, as her skills with the higher plants far exceed mine, and walked as much of the stream bed as we could, starting near the remains of a footbridge at NZ 424 389, and making our way downstream to Denemouth, where Castle Eden Burn joins the North Sea.  If my original intention was to better understand the burn by traversing space within the Dene, my first lesson concerned time: a week with some heavy rainfall separated my two visits and it was clear straight away that the Burn had been flowing during the week, with a fine layer of silt and mud spread across much of the surface, making parts of it slippery to walk upon.  There were standing pools of water at several points in the upper part of the Burn too.   Within a week the stream had come and gone, offering scant opportunities for any water-loving organism to establish.

We made our way along the Burn through the delicious silence of the forest.  The banksides were richly vegetated: masses of opposite-leaved golden saxifrage plus the mosses I described last time and many others, along with plenty of harts-tongue fern (Asplenium solopendrium).   Then, with a very clear demarcation, there was the stony stream bed with very little vegetation at all.    We looked hard for three plants, in particular, that I associated with the damp margins of streams, and which I had expected to see here: Verronica beccabunga (brooklime or water speedwell), Rorippa nasturtium-aquaticum (water cress) and Mentha aquatica (water mint).  None seemed to be present in any of the stretches we visited apart from a single sorry looking brooklime in the freshwater marsh at Denemouth .

What we did find, a little further downstream, was a pebble and gravel-dominated stretch with a straggly array of plants, all bent over in the direction of flow.   These included broad-leaved dock (Rumex obtusifolius), nettles (Urtica diocia), a few shoots of Himalayan balsam (Impatiens glandulifera) and some grasses.    Were I not standing on a dry stream bed I would have assumed that this was a bare piece of ground being colonised by typical ruderal species.  And that, I think, offers some insights into the ecology of Castle Eden Burn.   This is not a stream that occasionally dries out: it is a long-thin terrestrial habitat that is occasionally flushed through by water.   Welcome to north-east England’s premier wadi.


Rumex obtusifolius and other ruderal vegetation on the stream bed of Castle Eden Burn, August 2019.

This hypothesis really needs corroboration by a hydrologist, but the graph I showed in “Out of my depth …” shows that, despite flow being generally low,  episodes of high flow are scattered throughout the year, and I suspect that these keep the substratum mobile and, more important, stop organic matter accumulating to give amphibious plants an opportunity to establish.   The water table, too, I guess, is too far below the stream bed in between the spates to make it easy for plants to stay hydrated.   This is one of the main differences between Castle Eden Burn and the southern chalk streams, which are characterised by very stable flow regimes

From the point at which Blunt’s Burn enters Castle Eden Burn (NZ 436 396) there does seem to be permanent flow down to the sea.  Still, however, there was very little in-stream vegetation.  That was in contrast to the forest around us, which was floristically-rich (Heather has written more about this on her blog) and, on this warm summer morning, positively humming with bees and aflutter with butterflies.

A large embankment takes the busy A1086 over the Dene, the Burn passing through a long culvert at this point, after which there is a viaduct taking the coastal railway line across before the dene widens out into a large area of meadow just before it reaches the sea.   The stream’s path to the sea is, however, blocked by mine waste that was dumped from the coal mines that used to line the Durham coast.  This forces the stream to turn ninety degrees south for a few hundred metres before finding a way through and, gradually, trickling and percolating through the beach. The mines have all gone now and the sea is gradually eroding this compacted mass of waste.  Before the waste arrived, apparently, there was an area of saltmarsh at the mouth of the burn.   Now, there is a freshwater marsh, dominated by reeds (Phragmites australis).  When the mine waste finally goes, maybe the saltmarsh will return.  Meanwhile, Castle Eden Burn has no grand finale: it ends on a whimper not a bang.

We climbed a narrow, steep pathway up through gorse and brambles onto the clifftops overlooking these final stages of Castle Eden Burn to get a view that was, in light of all that had passed through my mind earlier, oddly symbolic.  The stream flowed almost due east until it encountered the bar, and the gentle arc which it then describes looks just like a question mark.   How ironic, I thought, for a stream that raises more questions than answers to sign off in that way ….


Denemouth, at the end of Castle Eden Dene, just above the point where the stream joins the North Sea.

A thousand little mosses …


Nature doth thus kindly heal every wound. By the mediation of a thousand little mosses and fungi, the most unsightly objects become radiant of beauty. There seem to be two sides of this world, presented us at different times, as we see things in growth or dissolution, in life or death. And seen with the eye of the poet, as God sees them, all things are alive and beautiful.
Henry David Thoreau (journal entry, March 13, 1842)

I was back in Castle Eden Dene earlier this week for my regular visit and, once again, encountered a dry stream bed.  This was no great surprise but, having written about the algae of dry river beds in earlier posts from Castle Eden Dene (see “When the going gets tough“ for the most recent instalment), I thought that I would focus on some of the other vegetation that I could see in and around the stream and, in particular, the bryophytes.   I asked Gaynor Mitchell, who wrote her MSc thesis on the bryophytes of the Dene, to come along and help me with these as my skills never really extended beyond those mosses and liverworts that live permanently submerged in streams and, as we have seen, there is rarely enough water in the burn here for such species to thrive.

There is a rich carpet of mosses on the woodland floor in much of Castle Eden Dene but, in the stream bed and its immediate environs, it is thalloid liverworts that are the most conspicuous bryophytes. Two species, in particular, stand out: the first is Conocephalum conicum, which has broad ribbon-shaped branches and an upper surface covered with pores – which just visible as light coloured dots to the naked eye.   The other is Pellia epiphylla, which was particularly noticeable on the top surface of boulders that are, I suspect, rarely covered, even when the burn is very full.   P. epiphylla had smaller thalli than C. conicum and, importantly, lacked the distinct pores on the upper surface.


Conocephalum conicum from Castle Eden Dene, July 2019. The pores are clearly visible on the thallus in the lower image.


Pellia epiphylla from the top of a boulder in Castle Eden Burn, July 2019

Alongside Pellia epiphylla on the boulder tops were shoots of the moss Thamnobryum alopercum.  The populations on top of the stones were rather non-descript to the naked eye, being stems growing horizontally across the rock surface. However, amidst these, we found a few of the upright stems which have a distinctly tree-like appearance.   We found more characteristic growths on the woodland floor nearby and my now-dated copy of Watson does, in fact, comment that this species has these two distinct habitats and also that it is a good indicator of calcareous conditions (for anyone who had not noticed the towering limestone cliffs in Castle Eden Dene, I presume?).   Lower down (and, thus, more frequently submerged), we saw Rhynchostegium confertum though this, too, is a species more often associated with terrestrial rather than aquatic habitats.  More significantly, the mosses I associate with streams in north-east England – Rhynchostegium riparioides, Fontinalis antipyretica and Leptodictyon riparium – are all missing from Castle Eden Burn.


Tree-like shoots of Thamnobryum alopercum from the forest floor in Castle Eden Dene in July 2019.  Growths on rocks in Castle Eden Burn were smaller but there were enough upright stems for it to be recognisable with the naked eye. 

Gaynor’s sharp eye spotted many other mosses and liverworts, though more in the woodland around the stream than in the stream bed itself.  As well as mosses and liverworts, the stream’s vegetation also consisted of a number of grasses and patches of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage.

The story that the vegetation is telling is, I would venture, that Castle Eden Burn is a shaded terrestrial habitat that is occasionally wet, rather than an aquatic habitat that is often dry.  I dug out an old account of the Winterbourne Stream, an intermittent stream in the chalk downlands of southern England for comparison, and found little overlap in the species recorded.   Care is needed for this comparison as the focus of the surveys is different (the Winterbourne account, for example, includes no bryophytes and spans perennial as well as intermittent sections) but there was a mix of genuinely aquatic and amphibious species, including Callitriche sp. and aquatic Ranunculus, which I did not see in Castle Eden Burn.    I suspect that Castle Eden Burn spends longer as a dry stream bed than the upper parts of the Winterbourne.  However, we also must remember that the Winterbourne data are now almost 50 years old, so that stream, too, may have changed much in the interim.

All this adds to my opinion that Castle Eden Burn – and the streams flowing through the other coastal denes in County Durham – are a unique and understudied habitat.  And that’s before I start thinking about the animal life here…


A patch of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage, on the bed of Castle Eden Burn, July 2019.


Berrie, A.D. & Wright, J.F. (1984).  The Winterbourne Stream.   pp.179-206.  In: Ecology of European Rivers (edited by B.A. Whitton).  Blackwell, Oxford.

Mitchell, G. (2015).  Bryophytes: changes in diversity and habitat in Castle Eden Dene (1975-2011).   Northumbrian Naturalist: Transactions of the Natural History Society of Northumbria 79: 39-66.

Watson, E.V. (1981).  British Mosses and Liverworts. Third Edition.  Cambridge University Press, Cambridge.

The intricate life of a colonial alga …


The annual Algal Training Course in Durham always has a field trip out to Cassop Pond, a small pond at the foot of the Permian Limestone escarpment in County Durham that has featured in a few of my posts over the years (see “A return to Cassop”).  This year, the group came back with some samples from the pond’s margins bearing a suspension of green dots just visible to the naked eye which, when examined under the microscope, turned out to be the colonial green alga Volvox aureus.  These are spherical, with the cells at the periphery, joined together by thin strands of protoplasm. The smaller colonies were scooting about, propelled by the pairs of flagellae borne by each of the cells that constitute the colony, whilst the larger ones (mostly “pregnant” with one or more daughter colonies) were sessile.


Volvox aureus colonies just visible to the naked eye in a drop of water from Cassop Pond, July 2019.   The drop is 13 millimetres across.


Colonies of Volvox aureus (each bearing daughter colonies) from Cassop Pond, July 2019.  Scale bar: 50 micrometres (= 1/20thof a millimetre).


A close-up of part of a colony of Volvox aureus from Cassop Pond, July 2019.  Scale bar: 20 micrometres (= 1/50thof a millimetre). 

Watching a Volvox colony swimming around under the microscope is a beguiling experience, but its movement is not random.  Consider: there may be a 1000 or more cells in the larger colonies, each with two flagellae.  If all beat their flagellae at random, the colony would not get anywhere, as the force in one direction would be cancelled out by forces in all other directions.   But Volvox colonies do actually move with intent.   Look closely at the individual cells in the photos below and you will see that each has a red-coloured eye spot (the light-detecting organelle actually lies beneath the red layer, which acts as a filter).   People with more patience than me have noticed that the eye spots in different parts of the colony differ in size, suggesting a level of organisation that may not be immediately apparent.  We also know that the daughter colonies tend to form at the posterior end of the colony (assuming “posterior” and “anterior” in a spherical colony are defined by the direction of travel) and also that only a small number of cells (larger than the others) are responsible for the division that produces these.

In theory, a spherical object is going to offer less resistance and so sink faster than an object of the same size that had a greater surface area : volume ratio. This should mean that they are not able to stay in the light-rich surface layers where they can photosynthesise and grow.   In practice, Volvox colonies are able to adjust their position by using their flagella but this requires them to pump some of the energy they have obtained from photosynthesis into the flagella’s motors.  Another advantage in Volvox’s favour is a relatively low density of the colony as a whole.  The individual cells are separated by strands of protoplasm which creates a lattice through which water can penetrate, so the overall density of the colony is closer to that of the surrounding water than would be the case if the cells were tightly packed.

Volvox is most often found in the summer in relatively nutrient rich lakes, where nutrients are sufficiently plentiful to support a rich crop of algae.  A motile colony that is not too dense is well-placed to adjust its position to stay in the surface layers and harvest the sunlight.  Moreover, the size of the colony probably means that it is too big for the filter-feeding zooplankton that grazes on the algae.   At the same time, however, Volvox begins to experience some of the problems associated with multicellular life (see references in “The pros and cons of cell walls …”).   As large multicellular organisms ourselves, a nuanced discussion about the pros and cons of multicellularity may seem to only have one possible outcome.   However, Volvox inhabits a world where plenty of single-celled organisms thrive and where a colonial lifestyle offers a small competitive advantage.  It means that it is quite happy drifting around at the time of year when many of us would like nothing better than to don swimming trunks and soak up some sun in a local pool.   Study algae for too long and you end up realising that only losers need to evolve.


Cells from a Volvox aureus colony from Cassop Pond, July 2019. You can see the red eye-spots in some of the cells in the left-hand image (bright-field) whilst the protoplasmic strands joining cells together can be seen in the right-hand image (phase contrast).   Scale bar: 10 micrometres (= 1/100thof a millimetre). 


Canter-Lund, H. & Lund, J.W.G. (1995).  Freshwater Algae: Their Microscopic World Explored.  Biopress, Bristol.

Reynolds, C.S. (1984). The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge.